The nature of the changes occurring in the skeleton during development, growth and aging may be related to the action of certain hormones on mature cells and/or their progenitors. In general, hormone induced changes in the function of existing (mature) cells are transient, whereas most sustained effects are due to an action on osteoprogenitor cells resulting in altered size of the mature population (Parfitt, 1976). Thus, a specific action of a hormone on the progenitor cell population could significantly influence skeletal physiology. Presently, the way in which differentiation of osteoprogenitor cells into osteoblasts is regulated is unknown. However, the factors involved must be identified before the involvement of the osteoprogenitor population in hormone or age-related changes in the skeleton can be defined. Thus, the long-term goal of these studies is to determine how osteoblast progenitor cell differentiation and osteoblast function is regulated. In this regard, the specific aims of the present proposal are to determine whether (1) the increase in alkaline phosphatase (AP) observed in cultured mouse calvarial bone cells treated with bovine parathyroid hormone (1-34) [bPTH-(1-34)] is due to an action on osteoprogenitor cells and/or osteoblasts by studying the effect of the hormone on transit of progenitors (labeled with tritiated thymidine) into mature cells (using AP as a marker) in organ culture and changes in AP activity in osteoblast and osteoprogenitor enriched cell cultures, 2) cyclic AMP mediates the action of bPTH-(1-34) on AP activity by identifying specific bPTH-(1-34) receptors, hormone stimulated adenylate cyclase and cyclic AMP-dependent protein kinase activities, and phosphorylation of specific protein substrates in cultures of responsive cells, and 3) the rise in AP activity is due to altered transcription and/or translation or a change in the rate of enzyme degradation by studying the effects of bPTH-(1-34) on AP in the presence of inhibitors of RNA and protein synthesis and by using an antibody prepared against skeletal AP.